I tend to think of bees both as not intrinsically smart but also not dumb.  If you provide handy sugar water, it's definitely easier to collect than going out to get it.  My recommendation is to use the amount of honey stored inside as your guide.  If it is quite or even mostly full, then I'd cut them off.

If they're still taking sugar water, though, I'd also be a bit concerned that they aren't actually using it themselves, but are being robbed.  When you open the hive, if there's very little honey stored, you might want to put on a robbing screen, or increase the strength of the hive.